There are an estimated 100,000 individuals with the loss of arms or hands in the United States alone and as many as 10,000 new amputees each year. Research has been carried out in the area of providing prosthetic limbs for many years. The result of this research has provided complex multiple degrees-of-freedom hands, which are too large and complex to be feasible in the marketplace. In contrast, a number of more commercially viable and affordable one-degree-of-freedom hands have been created. These prosthetic hands are combined with powered prosthetic elbows. The hands and elbows are driven by small electric motors. Command signals to drive the powered motors are provided by electrodes which receive electrical signals from the amputee's remaining muscles.
The practical one-degree-of-freedom hands or grips that have become commercially available have a number of shortfalls. One of these problems is the weight of the hands. Prosthetic hands on the market, which have a relatively high gripping force, weigh over 16 ounces. For hands weighing less than 13 ounces, the strength of the grip is cut in half.
Another significant problem with these hands is the safety features that are incorporated into the hands. It is difficult to incorporate safety features because of the desire to have hands with lower cost and complexity. For each safety feature that is incorporated into the hand, the weight of the hand may increase.
One specific problem is providing a hand or grip that can release in an embarrassing or dangerous situation. For example, suppose an amputee is getting off a public transit bus. To exit the bus, the amputee must “grip” the provided handrail. When the amputee steps away from the bus, the hand must then release its grip. It is possible that the hand may fail to release from the handrail because of a power failure or some other problem. In that situation, there must be a way to release the hand or it could endanger the amputee. If the hand does not release, the amputee could be pulled by the bus. Alternatively, the prosthetic hand, elbow and their assemblies might be disengaged from the amputee and taken with the bus as it drives away. Notwithstanding the danger, it can be very aggravating to lose the arm because it is difficult to retrieve and such prosthetic devices are very expensive. Other situations can also be imagined which would also present danger or embarrassment to the amputee if the hand does not release, such as descending stairs, riding an escalator, climbing a ladder, or getting stuck on a shopping cart in a public place.
It is desirable to provide a system to allow an amputee to quickly and safely release the prosthetic hand. Safety release devices have been developed, but most are not easy to operate or do not release the hand in a dangerous situation. For example, a prosthetic hand or grip manufactured by Otto Bock of Duderstadt, Germany, can only be released if 40 pounds of pressure is applied against the fingers and thumb of its prosthetic device. The Otto Bock Greifer, a work terminal device, also has a release mechanism which unmeshes interlocking gears near the load on the gripping surfaces. A lever moves the shaft of a transmission gear and separates the driving gear from a gear on the load side. This allows gears near the load to spin freely and release the gripping surfaces.
One major disadvantage with a mechanism that actually disengages the gears is that the delicate balance between the gears is disrupted. When the gears are re-engaged there is a certain amount of movement that must take place to force the gears together. This re-engagement of the gears produces wear and tear on the gears. As the gears wear, they do not operate as efficiently and their operation noise increases. A second major disadvantage to the release mechanism for disengaging the gears is that it is not easily accessible in an emergency because the fingers of the hand must physically be pulled apart, which is difficult for a one-armed or no-armed person.